Inductively heated cathode



Dec. 15, 1931. HELLMUND 1,836,966

INDUCTIVELY HEATED CATHODE Filed April 26, 1923 WITNESSES: INVENTOR a h d Rug/pEf/e mun 7: envelope.

Patented Dec. 15, 1931 UNITED STATES PATENT OFFICE RUDOLF E. HELLMUND, OF SWISSVALE, PENNSYLVANIA,. ASSIGN'OR T WESTING- HOUSE ELECTRIG & MANUFACTURING VANIA Application .flled April 26,

tion is to provide a cathode structure, in which electromagnetic induction as well as direct resistive effects may be used as the means for supplying the energy for heating the cathode to a temperature suitable for electron emisfl= sion and which will, at the same time, prevent interference between the electron stream and the magnetic field developed in connection with the inductive heating structure.

More specifically, my invention provides a "cathode structure in which an inductor coil is so magnetically interlinked with a secondary member through a relatively low reluctance magnetic path that the magnetic flux induces heating currents in'the cathode structure while being prevented from straying beyond the boundaries of the cathode structure into the electron stream.

In the design and operationot electron tubes, it is very desirable under certain conditions to employ a cathode having a truly, equipotential surface and to have a larger cathode surface than is available with the ordinary type of filament, such as is employed in many of the tubes of the prior art. It has also become a serious problem in some tubes to conduct the heavy, low voltage, heating current required by the filament type of cathode through the walls of the tube without damage to the seal or to the enclosing It has also become desirable in certain cases to employ alternating current, rather than low voltage direct current and to do this by means which will obviate noises in connected apparatus which may be produced by alternating current through magnetic modulation of the electron stream when such alternating current is used with conductively heated filament cathodes of theprior art.

Nit-h the above considerationin view, vats COMPANY, A CORPORATION or PENNSYL- INDUCTIVELY HEATED CATHODE 1923. Serial No. 634,881

tempts have been made heretofore to heat the cathodes of electron tubes by high-frequency inductive means, rather than by conductive means, placing the cathode material in inductive relation to an inductor, circuit carryin a small, high-potential, higlrfrequency current. So far as I am aware, however, all previous devices constructed for the purpose just stated have been adapted only to heating by high-frequency currents and have had such arrangement of parts that the magnetic flux producedby the inductor coil not only heatedthe cathode but also exerted an objectionable choking efiect upon the electron stream. Furthermore, the electrostatic field which occurs with thehigh voltage necessary to introduce sufficient high-frequency energy :for heating the cathode produces an equally objectionable choking and distorting effect uponthe electron stream of such prior inductively heated electron tubes as have come to my attention.

My invention in its specific details consists, therefore, of a cathode structure containing an inductor coil circuit, a secondary circuit, and a relatively low reluctance magnetic circuit, the inducing coil serving as a means of energy input, the secondary circuit serving for the conversion of induced electric current to'heat energy and for partial control of the position ofthe field of magnetic flux, and the magnetic circuit serving as a low reluctance magnetic path to restrain the magnetic flux within the cathode structure, thereby preventing interference with the electron stream. The whole construction is of such sizeas to be readily inserted within an anode and control electrode structure of usual design, thereby retaming the advantages of inductive heating and avoi ding the disadvantages previously encountered in inductive heating,

and the disadvantages of conductive heating. Other objects and structural detalls of my invention will be apparent :tromthe follow-' ingdescription and claims when read in Y other suitable material.

connection with the accompanying drawings, wherein Fig. 1 is a sectional view illustrating a device embodying my invention in its preferred form.

Fig. 2 is a plan view of the device shown in Fig. 1. t

In the drawings an evacuated glass container 1, mounted on a suitable standard base 2 maintains the necessary degree of vacuum for proper operation of 'the device. WVithin the container 1 is mounted a split anode structure 3 and a suitable control electrode structure 4, both being supported upon the customary press 5 through which pass the lead wires 6 and 7. Vithin anode 3'and con trol electrode 4 and concentric with them is mounted the cathode structure 8 carried by leads 10, 11 and 12, also extending through the press 5.

Cathode structure 8 consists, as shown, of a spool shaped core 13, around which is wound an inductor coil 14. Inductor coil 14 is insulated in any suitable way, as by a coating of low melting glass, by mica or by Surrounding the spool shaped core 13 and inductor coil 14 is a cylindrical metal cathode covering 15, which may be of iron or other suitable material and may be nickel plated and coated with a suitable earth metal oxide to increase its electron emission. Both core 13 and cylinder 15 may be of a magnetic material such as iron or of a non-magnetic metal depending upon the frequency of the current by which the heating energy is to be introduced.

In the structure as outlined, the plate 3' and the control electrode or grid 4 perform the customary functions of such elements in atriode tube, receiving and controlling the electron stream. The cathode structure 8 also serves the corresponding customary function of furnishing an electron stream.

This cathode, however, is not adapted to be heated by energy liberated as heat through the resistance drop encountered when a current is led through it from an outside source of current supply as is done with filament cathodes. Instead, in operation it is heated by currents flowing in it by reason of the alternating magnetic field produced by the contained inducing coil 14, and also by a small amount of heat developed by" the resistive drop through the conductor of the inducing coil. Some of the heat produced isliberated directly in the cathode cylinder 15 by the currents induced therein and is immediately effective for heating the electron emitting surface. Other heat is liberated in the core 13 by its induced currents and conducted to the electron emitting surface. If the device is to be operated on a low-frequency such as. for example, a commercial frequency of 60 cycles; the core 13 .and the cylinder 15 are preferably made of a fer-' rous material. The inductor coil 14 induces magnetic flux in the spool shaped core 13 which flux completes its path through the ferrous material of the cylindrical covering 15. The presence of this flux, in the cylinder 15 and the core 13, both of which are complete closed electric circuits, generates induced currents flowing in their body material. These currents rise to such a value that their effective ampere turns are substantially equal to the ampere turns in the inducing winding, and the energy converted into heat by them is proportional to their magnitude and to the resistance of the circuits formed by the material of core 13, and covering 15. These induced currents result in the liberation. of energy, which can be made of suliicient quantity to heat the whole cathode structure to a temperature at which it will emit a suitable electron stream. The necessary temperature depends uponthe presence or absence of a coating of earth metal oxide on the outer cathode surface, as is well known in the art.

In operation, the presence of the spool shaped core 13 of ferrous material furnishes a path of low-magnetic reluctance within the inducing coil 14, and the presence of the ferrous material cylindrical covering 15 g'ycs a low magnetic reluctance path completing the magnetic circuit. Therefore, most of the magnetic flux is carried by this low reluctance magnetic circuit.

By making the cathode cylinder 15 of a material that is electrically, as well as magneticaliy, conducting, I secure a double effeet in restraining the magnetic flux incident to the cathode. heating from reaching the space in which the electron flow from the cathode to the anode is intended to be controlled solely through the action of the grid 4.

First, the currents induced in the shortcircuited turn represented by the cathode cylinder 15 causes the flux which tends to reach out beyond the region of the cylinder to recede into the interior thereof in the same way as the short-circuited secondary winding of a transformer causes the major portion of the transformer flux to flow along the leakage path, so as not to be interlinked with the secondary winding. Second, by reason of the high magnetic permeability of the cathode cylinder, the resultant flux is deflected into the cylinder, thus additionally diverting the lines of flux away from the electron-discharge space. In this way, a combined electric and magnetic screening effect is obtained.

If tie electron emitting surface of the cathode is treated with the customary earth metal oxide, the most suitable temperature for electron emission is found to be between 700 degrees C. and 750 degrees C. This temperature also is very close to the critical temperature at which manyferrous materials lose their magnetic permeability.

This loss of magnetic permeability at a critical temperature canbe utilized to make the cathode structure of my invention selfregulating as to temperature. It is readily possible'to choose iron alloys whose critical point will lie at the same temperature'as' is found to be most suitable for electron emissionfrom earth metal oxide coated surfaces. The loss of permeability at this temperature can then be made to control the input of energy for heating.

During the time that the temperature of the ferrous material cathode structure remains below the critical temperature of the ferrous material of which it is composed, a low reluctance magnetic path is available through this ferrous material for the magnetic flux generated by the inducing coil, and therefore, the flux produced by a given current is sufiicient to induce a substantial current in the cathode structure. If the current through the device is held substantially constant as the temperature rises, by any suitable means, such as a high impedance transformer, the energy input into the cath ode will remain substantially constant until the temperature of the structure reaches the critical point of the ferrous material. When this occurs, the magnetic path increases in reluctance and in so doing cuts down the flux density of the magnetic field. Such reduction in flux density lowers the induct-ion applied to'the secondary member or coil of the structure (which is cylinder 15) and thereby causes less induced current to flow and less heat energy to be liberated until sufficient heat has been lost by radiation to lower the temperature of the ferrous material cathode structure to a point where its magnetic permeability is substantially increased.

The cathode structure as a whole, will therefore, attain an equilibrium temperature which is determined by the critical temperature of the ferrous material. It may be noted that the lowering of reluctance of the ferrous material structure and the consequent lowering of the induced magnetic flux, lowers the. impedance which the device offers to the input current andtherefore, a constant current device may be desirably included in the circuit to make the self-regulating feature effective. V

In the operation of the tube, the presence of current in the cylindrical covering 15 produces counter-magnetomotive force in the space without the cylinder which tends to reduce the value of the flux in the space surrounding the cathode. In this manner, the magnitude of the flux which traverses the space surrounding the outer surface of cathode covering 15, from which the electron stream is emitted, is kept to a minimum value,

thereby reducing to a negligible value the effect of magnetic modulation upon the electron stream, and at the sametime reducing tube noises in associated apparatus, traceable to such magnetic modulation.

This structure may also be employed with high frequency alternating current as the source of supply of the heating energy. -With such high frequency current the coil 14 and covering 15 may or may not be of magnetic material. In the event that it is of magnetic material a reduced number of turns is required'in inductor coil 14, over those required forthe same frequency when another metal is used for core 13 and cylinder 15. In the event that magnetic material is used, the magnetic relations are substantially the same as in the case of low frequency cur By the construction as before described and by modifications thereof which will be readily apparent to those skilled in the art, it

.is possible to produce ahot cathode device in which the cathode is heated by induced current under conditions which permit of'the separation of the electron stream from the magnetic field of flux which is necessary for generating the induced currents which do the. heating, thereby producing a device which has the advantages of the inductively heated cathode and avoids the disadvantages previously encountered in such devices.

While I have shown only one typical embodiment of my inventionin the accompanyingdrawings, it is capable of various changes and modifications without departing from the spirit thereof and it is desired, therefore, that only such limitations shall'be imposed thereon as are indicated in the prior art or in the'appended claims.

I claim as my invention j 1. A hot cathode device comprising a uni potential cylindrical cathode,.having a surface capable of electron emission, an inductor coil within said cylindrical cathode, and an electrically conducting core within said cylindrical cathode and said coil.

2. A hot cathode for an electron discharge device, comprising a cathode cylinder, having-a surface capable of electron emission, an

inductor coil contained within said cathode cylinder, and a magnetic core within said coil and said cathode cylinder.

3. An electron discharge device, comprising an inductively heated, cylindrical cathode, an inductor coil, and a magnetizable core within said inductor coil, said cathode surrounding both said inductor'coil'andsa id magnetizable core.

. a. Anelectron discharge'device, compris- Tao ing anfinductivelyvheated, unip'otential cathode, aninductor-coilWithin thecathode and "a magnetizable core, Within said .coil,;sa1d cathode being composed'of a cylinder of electrically conductingmaterial.

.5. A hot cathode for an electron discharge device, comprising a cylindrical unipotential cathode structure and inductive heating means'therefor, said heating means comprising aninductor coil for generating magnetic flux, and a-highly permeable, electrically conductive path for said flux, said inductor'coil and said magnetic path, being containedwithin'the evacuated space of said electron discharge device.

6. A space discharge device, comprising an inductively heated unipotential cathode for emitting. an electronstream, an anode, a control electrode and an evacuated container, said cathode comprising a metallic 'core, an insulated inductor coil WOIllld thereonand a conductive cylinder surrounding said core and said inductorco'iLsaid cylinder having its outer surfacetrea'ted to increase its electron emission.

7. A spaced discharge device, comprising an inductively heated unipotential cathode, emitting an electron-stream, an anode,-a control electrode, and an evacuated container, said cathode comprising 'aniron core, an insulated inductor coil woundithereon, and an iron cylinder surrounding said core and said inductor coil, said cylinder having its outer surface treated to increase its electron emission. r

8. Ina hot-cathode-electron-discharge device, a cathode comprising an electronemitting member,'means for supplying-energy to heat said'member to a temperature of electron emission, said cathode embodying means responsive to temperature variations for regulating the energy inputintosaid cathode.

9. In' a hot-cathode-electron discharge device, a cathode comprising an electron-emitting member adapted to operate at an eleing energy thereto, and means embodied in said cathode for producing areduction in the heat energy supply in response to a temperature increase above a predetermined Value, and vice versa. I f

10. A thermionically emisssive cathode comprising a structure of ferro-magnetic material,'means for inducing a magnetic flux in saidmaterial, and a thermionically emissive coating so related to said ferro-magnetic material'as to be in good thermally conductive relation therewith, thetemperature at Which the term-magnetic"property;of said ferromagnetic materials substantially disappears being substantially equal to the-maximum safe operating temperature for said thermionically emissive material.

11. An inductively heated cathode comprising a structure Otferro-magnetic matevated temperature, means for supplying heatamameee 

